In search for a more specific method for met 5-enkephalin assay, a procedure based on fragmentation of the enkephalin by dipeptidyl aminopeptidase and subsequent analysis of the resultant dipeptides and amino acid was explored. In this method, met 5-enkephalin can be specifically measured without interference from leu 5-enkephalin. The method is easily applicable to the tissue in which there is a very low degree of molecular heterogeneity such as brain tissue. However, the method may be applicable to tissue, where there is a high degree of molecular heterogeneity, only after extensive prepurification.